TAF-containing and TAF-independent forms of transcriptionally active TBP in vivo

Transcriptional activity in yeast strongly correlates with promoter occupan cy by general factors such as TATA binding protein (TBP), TFIIA, and TFIIB, but not with occupancy by TBP-associated factors (TAFs). Thus, TBP exists in at least two transcriptionally active forms in vivo. The TAF-containing form corresponds to the TFIID complex, whereas the form lacking TAFs corres ponds to TBP itself or to some other TBP complex. Heat shock treatment alte red the relative utilization of these TBP forms, with TFIID being favored. Promoter-specific variations in the association of these distinct forms of TBP may explain why only some yeast genes require TFIID for transcriptional activity in vivo.